1. Field of the Invention
This invention relates to novel chemical compounds, useful as optical indicators of cytochrome P450 activity, and especially to fluorogenic indicators of cytochrome P450 activity. More specifically, the invention relates to ether-containing compounds of the generic structure Y-L-Q, and to methods for assaying substrates and inhibitors of cytochrome P450 enzymes using these compounds in traditional assay formats, as well as in high and ultra high throughput screening formats.
2. Description of the Related Art
The cytochrome P450 enzyme (CYP450) family comprises oxidase enzymes involved in the xenobiotic metabolism of hydrophobic drugs, carcinogens, and other potentially toxic compounds and metabolites circulating in blood. It is known that the liver is the major organ for xenobiotic metabolism, containing high levels of the most important CYP450 mixed-function oxygenases. There are numerous human P450 enzyme sub-families, often termed “isozymes” or “isoforms.” Those of the CYP 3A4, CYP 2D6, CYP 2C, CYP 2A1 and CYP 2E1 subfamilies are known to be important in drug metabolism. See, e.g., Murray, M., 23 Clin. Pharmacokinetics 132–46 (1992). Of these isoforms, CYP 3A4 is by far the major isoform in liver and the small intestines, comprising 30% and 70% respectively of the total CYP450 protein in those tissues. Based primarily on in vitro studies, it has been estimated that the metabolism of 40% to 50% of all drugs used in humans involve CYP 3A4 catalyzed oxidations. See Thummel, K. E. & Wilkinson, G. R., In Vitro and In Vivo Drug Interactions Involving Human CYP 3A, 38 Ann. Rev. Pharmacol. Toxicol., 389–430 (1998).
Efficient metabolism of a candidate drug by a CYP450 enzyme may lead to poor pharmacokinetic properties, while drug candidates that act as potent inhibitors of a CYP450 enzyme can cause undesirable drug-drug interactions when administered with another drug that interacts with the same CYP450. See, e.g., Peck, C. C. et al., Understanding Consequences of Concurrent Therapies, 269 JAMA 1550–52-(1993). Accordingly, early, reliable indication that a candidate drug interacts with (i.e., is a substrate or inhibitor of) a CYP450 may greatly shorten the discovery cycle of pharmaceutical research and development, and thus may reduce the time required to market the candidate drug. Consequently, such earlier-available, reliable CYP450 pharmokinetic information may result in greatly reduced drug development costs and/or increased profits from earlier market entrance. Furthermore, such earlier-available, reliable CYP450 pharmokinetic information may allow a candidate drug to reach the public sooner, at lower costs than otherwise feasible. Accordingly, extensive pharmacokinetic studies of drug interactions in humans have recently become an integral part of the pharmaceutical drug development and safety assessment process. See, e.g., Parkinson, A., 24 Toxicological Pathology 45–57 (1996). Methodologies are therefore desired that will allow for (1) the more rapid acquisition of information about drug candidate interactions with CYP450 enzymes, earlier in the drug discovery process than presently feasible, and hence will allow for (2) the earlier elimination of unsuitable compounds and chemical series from further development efforts.
The need for information regarding drug candidate/CYP450 interactions has created a concurrent need for assays sensitive enough to test, in a cost-effective manner, vast arrays of compounds for interactions with the major human CYP450 enzymes involved in drug metabolism. Certain known techniques, including (1) CYP450 inhibition assays in which the metabolism of known CYP450 metabolite in the presence of the test compound, followed by quenching of the enzyme reaction and analysis of the extent of metabolism, (2) CYP450 metabolism of radioactively labeled test compound analogues, and (3) in vivo “cassette” dosing of animals (usually rats, dogs, or monkeys), see Berman, J. et al., Simultaneous Pharmacokinetic Screening of a Mixture of Compounds in the Dog using API LC/MS/MS Analysis for Increased Throughput, 40 J. Medicinal Chemistry, 827–29 (1997), are not amenable to adaptation to miniaturization, or to the other requirements of high or ultra high throughput screening.
However, optical assays employing, for example, chromophores or luminescent phenols, and especially fluorescence-based assays are amendable to adaptation to miniaturization and high or ultra high throughput screening. Particularly, fluorescence-based assays have been used in pharmacokinetic studies of drug interactions in humans, more particularly in assays involving human hepatocyte cultures, where the number of available cells is severely limited. See Donato, M. T. et al., 213 Anal. Biochem. 29–33 (1993).
Specifically, fluorogenic cytochrome P450 substrates have been commercially available for a number of years from, for example, Molecular Probes, Inc. (Eugene Oreg.), SIGMA (St. Louis, Mo.), and more recently, GENTEST Corp. (Woburn, Mass.). Generally, these known fluorogenic CYP450 substrates are ether derivatives of well-known phenoxide type fluorophores, including: 7-hydroxycoumarin, fluorescein, and resorufin. Thus, generally, the CYP450 enzymes will catalyze a dealkylation reaction and convert the relatively non-fluorescent ether substrate into a relatively more highly-fluorescent phenoxide-containing product.
However, even the most recently developed fluorogenic CYP450 substrates either have relatively poor kinetics, or the enzymatic products do not have the desired physical and optical properties to allow reduction of the amount of enzyme needed to levels that would make large scale screening affordable and feasible. More specifically, these fluorogenic CYP450 substrates exhibit relatively poor turnover rates, poor aqueous solubility, low extinction coefficients and quantum yields, and/or weak fluorescence of the resultant phenolic dye. Furthermore, certain of these fluorogenic CYP450 substrates are excited in the ultraviolet, as opposed to visible, spectrum and therefore their signals are often masked by background stemming from the unreacted test compound. Finally, most of these fluorogenic CYP450 substrates are not specific for the CYP450 isozyme they are meant to detect, and therefore cannot be used for measurement in human liver microsomal preparations, a preferred analytical method that avoids potential artifacts caused by the alternative method of using an insect cell microsomal preparation. See Palamanda J. R. et al., Validation of a rapid microtiter plate assay to conduct cytochrome P450 2D6 enzyme inhibition studies, 3 Drug Discovery Today, 466–470 (1998). For these and other reasons, there exists an unfulfilled need for optical, and especially fluorogenic, CYP450 substrates that exhibit CYP450 isozyme-specificity, improved kinetics, and yield enzymatic products having improved physical and optical properties for use in the screening of CYP450/drug candidate interactions, especially for use in high or ultra high throughput screening, and as part of the drug discovery process.